This invention relates generally to high-speed line printers employing towel ribbons, and more particularly to an improved method and apparatus for detecting the edge of such towel ribbons.
Conventional high-speed line printers, such as the Impact 5000 Series printers manufactured by Documation Incorporated (now StorageTek Printer Corporation), a subsidiary of Storage Technology Corporation, are capable of operating at rates in excess of 3,000 lines per minute, and typically employ selected indicia producing characters which strike an inked ribbon which is disposed adjacent to the paper to be printed. The inked ribbon may be on the order of 14-18 inches in width, and is typically fabricated from nylon cloth, silk, polyester film or the like. Such ribbons often provide several million lines of printing before the ink contained therein is depleted to the point where the printing indicia is not of a sufficient density.
Different colored ribbons are typically used for different applications. For example, ribbons containing carbon black are often used for applications which require a neat, polished look. Purple ribbons, on the other hand, are cheaper and last longer than carbon black ribbons, but their print quality is not as good. As a result, purple ribbons are most often used for program listings, shipping listings, material listings, and the like. Also used in high-speed line printers for "no carbon required" or NCR-type applications are inkless ribbons.
As is well known, conventional high-speed line printers utilize at least one ribbon edge sensor, most often of the photoelectric variety, in order to provide positioning information for the towel ribbon. One such photoelectric ribbon-edged detector assembly is disclosed in U.S. Pat. No. 4,164,911, issued to Albert N. Nicholson, and assigned to Engineering Systems Corporation. This assembly includes a pair of photoelectric detectors secured to a portion of the upper frame assembly, and slightly offset longitudinally so that their sensing apertures may straddle the plane in which the edge of the towel ribbon is to be disposed. As the edge of the towel ribbon passes between the light sensors and their corresponding light sources, one or the other of the sensors will be blocked or actuated as the ribbon edge wanders while the towel ribbon is being unwound from one spool mechanism and wound onto the other one. Changing signals from the light sensors are employed by the control logic in a conventional manner to control a motor which in turn selectively controls the axial position of the hubs supporting the ribbon spools. In such a manner, the photoelectric edge sensing signals are used to rectify the wrap of towel ribbons which may have been poorly wrapped while being used in a high-speed line printer.
As noted hereinabove, different colored towel ribbons are used for different applications. However, prior art photoelectric ribbon edged detector assemblies such as that described in U.S. Pat. No. 4,164,911 suffer from a common disadvantage in that the accuracy of ribbon edge detection varies in any ribbon not containing carbon black, such as the aforedescribed purple ribbons and inkless ribbons. Because the ribbon edge cannot be accurately and repeatedly detected, highspeed line printers which utilize such prior art ribbon edge detector assemblies may suffer from unprinted characters when the edge of the ribbon uncovers the print hammers. It would, therefore, be desirable to provide an improved optical ribbon edge sensor which would be capable of operating with various colored towel ribbons.
Another disadvantage of the structure of typical prior art ribbon edge detector assemblies resulted from the deep throat provided between the light sensors and their corresponding light sources. One such ribbon edge sensor having a deep throat is manufactured by HEI Inc., Chaska, Minn., as model number HEI 126--1. It can be readily appreciated that any such sensor arrangement having a deep throat complicates loading and unloading of a towel ribbon in that the operator must insure proper seating of the towel ribbon within the throat area of the ribbon edge detector assembly. Accordingly, it would also be desirable to provide an improved ribbon edge detector assembly having a shorter throat, thereby promoting an easier or self-loading feature.